Electronic components such as semiconductor chips or the like mounted on electric or electronic devices including electrical power equipment, various devices such as personal computers or the like generate heat in some extent when used. When the electronic components are excessively heated, the performance thereof is lowered, or the lifetime thereof is shortened. The technology to cool the electronic components is therefore an important technical issue. There is known as a method of cooling heat generating parts, for example, a cooling method by means of ventilating air in the interior of devices by fans to cool same, a cooling method by means of attaching heat dissipating fins to the heat generating parts to cool same, or a cooling method by means of using a heat pipe to cool same.
Metallic materials such as copper, aluminum or the like which has a higher heat conductivity, ceramics such as carbon material, aluminum nitride or the like which has a higher heat conductivity are preferably used as the fins which are attached to the heat generating parts. However, heat generating parts are not sufficiently cooled by the cooling method by means of attaching heat dissipating fins thereto. Thus, a plate type heat pipe comes to be used to cool heat generating parts.
The heat pipe is briefly described hereunder. The heat pipe includes a container having a hermetically sealed hollow portion, i.e., cavity therein, and a working fluid enclosed within the container. A heat is transferred from a heat absorbing portion to a heat dissipating portion, which is apart each other, by means of phase transition and movement of a working fluid accommodated within the hollow portion. The interior of the container is kept under the reduced pressure by means of evacuating in vacuum in order to facilitate phase transition of the working fluid.
When a portion of the container constructing the heat pipe is heated, the heat is transferred to the working fluid existing in the corresponding portion in the hollow portion (which portion is called as a heat absorbing side or heat absorbing portion), thus the working fluid is vaporized by the heat. Thus produced vapor of the working fluid flows within the hollow portion to the portion (which portion is called as a heat dissipating side or heat dissipating portion) of the heat pipe to which heat dissipating fins are attached, for example, and then the vapor is cooled enough in the portion to be condensed to a liquid phase. The working fluid thus condensed back to the liquid phase flows back (circulates) to the heat absorbing side by the function of gravity or the like. The heat is thus transferred by means of phase transition and movement of the working fluid as described above.
To secure the continuous circulation of the working fluid, the heat absorbing side is in general disposed below the heat dissipating side. When thus disposed, the working fluid transferred back to the liquid phase in the heat dissipating side flows back to the heat absorbing side by gravity. However, for example such electric or electronic devices as a note book type personal computer is largely inclined when used, or sometimes the personal computer is placed upside down according to the conditions to be used. Under those conditions, such a case actually happens that the working fluid hardly flows back by gravity.
In order to solve the above-mentioned problem, there is disposed a wick having a capillary action in the interior of the heat pipe (hollow portion). Metal mesh or metal wire is used as the wick, and in addition, fine longitudinal grooves are formed in the inner wall of the hollow portion to perform the capillary action.
Aluminum material, copper material, stainless steel material or the like is usually used as the material of the container forming the heat pipe. However, the container as a whole is not necessarily formed by the same material. Different materials may be used as combined to form the container. In addition, water, substituted Freon, alcohol or the like may be used as the working fluid.
Heat generating parts to be cooled are, typically for example, semiconductor chips or the like. The heat generating parts such as semiconductor chips are generally used in a fashion as mounted on a printed wiring board. As a cooling method of the heat generating parts, there is a favorable cooling method in which a plate type heat pipe and the printed wiring board are disposed face to face each other in such manner that the heat generating parts mounted on the printed wiring board are caused to contact with the facing surface of the heat pipe. The heat generating parts may directly contact with the surface of the heat pipe. The heat generating parts may contact with the surface of the heat pipe through a heat transfer grease or the like placed therebetween to reduce the contact resistance. In some case, the heat generating parts are joined to the surface of the heat pipe by soldering or the like.
However, the heat generating parts such as semiconductors or the like are small in size. Accordingly, even though the heat generating parts are caused to contact with the plate type heat pipe, the contacting area thereof is small. As a result, when a heat generating density of the heat generating part is high, the so-called dry out phenomenon is easily caused to occur in the corresponding portion or the vicinity thereof within the heat pipe to which the heat generating part is connected, in which phenomenon the working fluid is vaporized out and no working fluid in liquid phase exists in the portion.
When the so-called dry out phenomenon actually occurs, the heat generated by the heat generating parts is not fully transferred to the working fluid in the interior of the heat pipe, thus the heat generating parts are not effectively cooled to result in causing a trouble.